PDF.co uses a credit-based system. The credits refer to a certain amount it takes to process an API request.
The API functions/modules have their own predefined number of credits. Depending on the API function, these credits may be deducted per document page or per call. For example, a Credit Balance Check takes credits per call, while the PDF Merge takes credits per page.
You can find the list of functions and their corresponding credits here https://app.pdf.co/subscriptions/credits_pricing